Some types of non-volatile semiconductor memory devices are already proposed and many of them provide the structure similar to ordinary MIS FETs. A typical type of memory device provides many trap charges within the gate insulating film and stores binary states represented by changes of gate threshold voltage caused by charges in these traps. The memory device of this type is disclosed, for example, in the U.S. Pat. No. 3,549,911. Other typical types of such memory devices are memories providing a floating gate for storing charges, which are disclosed, for example, in the U.S. Pat. Nos. 3,500,142; 3,797,000; and 3,825,946.
For these non-volatile memory devices, two kinds of mechanisms are utilized for injection of charges to the charge traps or to the floating gate for data writing. One is the tunnel injection method where charges are injected through very thin insulating film by means of the tunnel effect and the other is the avalanche injection method where hot carriers generated by the avalanche phenomenon are injected through comparatively thick insulating film. Both of these methods are disclosed in the cited U.S. patents.
The tunnel injection method has a disadvantage in that the insulating film thickness, even in the case of maximum thickness, through which the charges pass by means of the tunnel phenomenon, is only about 100 .ANG. and such ultrathin insulating film is inferior in the reproducibility thereof, and tends to have many pin-holes which shortens the charge storing period. In the case of the avalanche injection method, the charge injection is possible through the thickness of insulating film even when it has a thickness of 1000 .ANG.. However, when injection of avalanche hot carriers is repeated, the insulating film may easily be damaged so as to adversely effect the charge storing characteristic. Therefore, the number of repetitions of write operation is liable to be restricted and this tendency is undesirable for memory devices. A further serious disadvantage of the avalanche injection system is that application of voltage as high as, for example, 35 volts, is required for write operation. A reduction of write voltage is still required as an essential condition even for the tunnel injection system.
Various problems explained above result from such an essential process of write operation in the existing memory devices that charges are injected through the insulating film, and therefore it is difficult to completely eliminate such problems so long as such write operation is utilized.
In addition to the typical non-volatile memory device diclosed in the above prior art devices, also known is a non-volatile memory device utilizing remnant polarization of a gate insulating film consisting of ferroelectric material, which is disclosed, for example, in the U.S. Pat. No. 2,791,760. In this case, the write voltage is extremely high. Moreover, the ferroelectric material is not yet fully investigated for adaptability to a semiconductor process, and this memory device is still far from a practical application. It is desirable for obtaining practical memory devices to utilize the structures of silicon (Si)-silicon dioxide (SiO.sub.2) or silicon nitride (Si.sub.3 N.sub.4) which are obtained in the prior art, as the structure of semiconductor-insulator.